How to free reserved space on ext2/3/4 partitions

Linux by default, set aside around 5% of the total space in a partition for the root user and system service. For systems partition like /, this does make sense, since you won't be able to login if your / is full, but for other partitions, it does not seems that the reserve will help in any way.

You can find how much is being set as reserved by running:

$ sudo tune2fs -l /dev/sda1 | grep 'Reserved block count'
Reserved block count:     27709

So, for ext3/4 partitions, you can reclaim that reserve space by setting the reserve count to 0:

$ sudo tune2fs -m 0 /dev/sda1

Reserved block count will be 0% after the above command

$ sudo tune2fs -l /dev/sda1 | grep -i 'Reserved block count'
Reserved block count:     0

You can now fully enjoy the maximum space available for your partition.

Reset openldap default admin password

This usually will be used during the installation of ldap, or when you have forgotten your ldap admin password. It took me quite a while to find these, so thanks to these article, that pointed me to the right direction. Here is how i did it:

1. Find the root dn account and root dn password hash: 

   $ sudo ldapsearch -H ldapi:// -LLL -Q -Y EXTERNAL -b "cn=config" "(olcRootDN=*)" dn olcRootDN olcRootPW | tee ~/newpasswd.ldif

2. You can see the above info right away since we are using tee (tee is for writing to new file, and show to stdout at the same time). This is how it looks like: 

   $ cat newpasswd.ldif
   dn: olcDatabase={1}hdb,cn=config
   olcRootDN: cn=admin,dc=ubuntu-ldap,dc=com
   olcRootPW: {SSHA}CS9o0OVuD4YOj1eFNf4q6eqSe8O4MBMy
   

3. Generate a new password for the admin, and append it to the newpasswd.ldif. -h is the flag to specify the scheme, and you can know the scheme by referring to the newpasswd.ldif file that we have generated, in this case {SSHA}: 

   $ sudo slappasswd -h {SSHA} >> newpasswd.ldif
   New password:
   Re-enter new password:
   

4. Edit the newpasswd.ldif, so that it will look like below (just comment olcRootDN, add changetype and replace, and change the oldRootPW to the one we generated in step 3:

   dn: olcDatabase={1}hdb,cn=config
   #olcRootDN: cn=admin,dc=ubuntu-ldap,dc=com
   changetype: modify
   replace: olcRootPW
   olcRootPW: {SSHA}CS9o0OVuD4YOj1eFNf4q6eqSe8O4MBMy
   

5. Implement the password change using ldapmodify command, where the flags are -H is for ldap uri, -Y for the SASL mechanism and -f for reading the input from file: 

   $ sudo ldapmodify -H ldapi:// -Y EXTERNAL -f ~/newpasswd.ldif

6. Test the new password by listing the entries using ldapsearch, making sure that the new password is working: 

   $ ldapsearch -h localhost -b "dc=ubuntu-cacti,dc=com" -D "cn=admin,dc=ubuntu-cacti,dc=com" -W

That should be it, your admin user is now having new shiny password.

Check postfix email queue, and how to clear emails in there

To check the queue, we use a command called mailq, example as below:

$ mailq | tail

2024F98A61     3505 Mon Apr 27 18:16:17  support.us@something.com
(delivery temporarily suspended: Host or domain name not found. Name service error for name=smtp.something.com type=AAAA: Host not found)
                                         support.us@somethingelse.com
231EC98AED     3525 Fri Apr 24 16:48:04  support.us@something.com
(delivery temporarily suspended: Host or domain name not found. Name service error for name=smtp.something.com type=AAAA: Host not found)
                                         whoami@somethingelse.com

To clear the email in the queue, a command called postsuper is used, as below:

$ sudo postsuper -d ALL

To check the status of deletion, you can refer to the maillog, in below example, 15 messages have been deleted:

$ sudo tail /var/log/maillog

Apr 27 19:44:02 au-mel-rh-dev-3 postfix/smtp[6927]: 46F5598AF7: to=, relay=mta5.am0.yahoodns.net[98.138.112.37]:25, delay=2568, delays=2562/0.02/4.3/2, dsn=2.0.0, status=sent (250 ok dirdel)
Apr 27 19:44:02 au-mel-rh-dev-3 postfix/qmgr[5985]: 46F5598AF7: removed
Apr 27 19:44:04 au-mel-rh-dev-3 postfix/smtp[6925]: ABA8B98AFA: to=, relay=mta5.am0.yahoodns.net[98.138.112.34]:25, delay=1624, delays=1617/0.02/4.4/3.3, dsn=2.0.0, status=sent (250 ok dirdel)
Apr 27 19:44:04 au-mel-rh-dev-3 postfix/qmgr[5985]: ABA8B98AFA: removed
Apr 27 19:49:27 au-mel-rh-dev-3 postfix/postsuper[7913]: Deleted: 15 messages

xrdp is not accessible with "X server -- no display in range is available" error

From rdp client, it is showing below error:

Checking on the log, this is the error:

$ sudo tail -f  /var/log/xrdp-sesman.log
...
[20150420-09:55:01] [INFO ] scp thread on sck 7 started successfully

[20150420-09:55:02] [INFO ] ++ created session (access granted): username mrapp, ip 10.157.4.55:53456 - socket: 7

[20150420-09:55:02] [INFO ] starting Xvnc session...

[20150420-09:55:02] [ERROR] X server -- no display in range is available

Searching on the web, I stumbled upon a site. Below are the steps that I learned to solve the above issue from the site:

1. Edit /etc/xrdp/sesmain.ini
2. Change MaxSessions from 10 to 100 

$ sudo diff -u {~,/etc/xrdp}/sesman.ini
--- /home/mrapp/sesman.ini      2012-10-29 20:58:30.000000000 +0700
+++ /etc/xrdp/sesman.ini        2015-04-20 09:56:31.277342575 +0700
@@ -13,7 +13,7 @@


 [Sessions]
 X11DisplayOffset=10
-MaxSessions=10
+MaxSessions=100
 KillDisconnected=0
 IdleTimeLimit=0
 DisconnectedTimeLimit=0

3. Restart xrdp

   $ sudo /etc/init.d/xrdp restart

Try to connect with your rdp client, it should work now.

Connecting to your machine using vnc

To setup and use vncserver in redhat flavored distro, please follow below steps:

1. Install the package:

   $ sudo yum install tigervnc-server tigervnc

2. Set a password for a user that you want to use to login to the vnc server:

   $ vncpasswd

3. Start the vncserver:

   $ vncserver

4. Check the display number of your newly created vnc session:
   

   $ vncserver -list
   TigerVNC server sessions:
   X DISPLAY #     PROCESS ID
   :1              9168

5. Check the port that your vnc session is using:

   $ sudo netstat -tulpn | grep 9168
   tcp        0      0 0.0.0.0:5901                0.0.0.0:*                   LISTEN      9168/Xvnc
   tcp        0      0 0.0.0.0:6001                0.0.0.0:*                   LISTEN      9168/Xvnc
   tcp        0      0 :::6001                     :::*                        LISTEN      9168/Xvnc

6. Test your vncserver locally (only if you are using GUI, else test it from the other machine with vncviewer installed), using vncviewer (belongs to tigervnc package):
   

   $ vncviewer :1

To connect from a redhat flavored linux distro client:

1. Install the tigervnc package:
   

   $ sudo yum install tigervnc

2. Run the vncviewer: vncviewer :

   $ vncviewer 192.168.0.2:1
   

3. If you are behind firewall, you need to open port 5901 for the client to get through.
4. If firewall is not in your control, you can always use ssh to port forward port 5901 to 5901 in your localhost, by: 

$ ssh -L 5901:localhost:5901 vncserver.ip.address

and in other terminal, run

$ vncviewer localhost:1

Once you are done, you can kill the server by running:

$ vncserver -kill :1